Emissions regulations for internal combustion engines have become more stringent over recent years. Environmental concerns have motivated the implementation of stricter emission requirements for internal combustion engines throughout much of the world. Governmental agencies, such as the Environmental Protection Agency (EPA) in the United States, carefully monitor the emission quality of engines and set acceptable emission standards, to which all engines must comply. Generally, emission requirements vary according to engine type. Emission tests for compression-ignition (diesel) engines typically monitor the release of diesel particulate matter (PM), nitrogen oxides (NOx), and unburned hydrocarbons (UHC). Catalytic converters implemented in an exhaust gas after-treatment system have been used to eliminate many of the pollutants present in exhaust gas. However, to remove diesel particulate matter, typically a diesel particulate filter (DPF) must be installed downstream from a catalytic converter, or in conjunction with a catalytic converter.
A common DPF comprises a porous ceramic matrix with parallel passageways through which exhaust gas passes. Particulate matter subsequently accumulates on the surface of the filter, creating a buildup which must eventually be removed to prevent obstruction of the exhaust gas flow. Common forms of particulate matter are ash and soot. Ash, typically a residue of burnt engine oil, is substantially incombustible and builds slowly within the filter. Soot, chiefly composed of carbon, results from incomplete combustion of fuel and generally comprises a large percentage of particulate matter buildup. Various conditions, including, but not limited to, engine operating conditions, mileage, driving style, terrain, etc., affect the rate at which particulate matter accumulates within a diesel particulate filter.
Accumulation of particulate matter typically causes backpressure within the exhaust system. Excessive backpressure on the engine can degrade engine performance. Particulate matter, in general, oxidizes in the presence of NO2 at modest temperatures, or in the presence of oxygen at higher temperatures. If too much particulate matter has accumulated when oxidation begins, the oxidation rate may get high enough to cause an uncontrolled temperature excursion. The resulting heat can destroy the filter and damage surrounding structures. Recovery can be an expensive process.
To prevent potentially hazardous situations, accumulated particulate matter is commonly oxidized and removed in a controlled regeneration process before excessive levels have accumulated. To oxidize the accumulated particulate matter, exhaust gas temperatures generally must exceed the temperatures typically reached at the filter inlet. Consequently, additional methods to initiate regeneration of a diesel particulate filter may be used. In one method, a reactant, such as diesel fuel, is introduced into an exhaust after-treatment system to initiate oxidation of particulate buildup and to increase the temperature of the filter. A filter regeneration event occurs when substantial amounts of soot are consumed on the particulate filter.
A controlled regeneration can be initiated by the engine's control system when a predetermined amount of particulate has accumulated on the filter, when a predetermined time of engine operation has passed, or when the vehicle has driven a predetermined number of miles. Oxidation from oxygen (O2) generally occurs on the filter at temperatures above about 400 degrees centigrade, while oxidation from nitric oxides (NO2), sometimes referred to herein as noxidation, generally occurs at temperatures between about 250 C and 400 C. Controlled regeneration typically consists of driving the filter temperature up to O2 oxidation temperature levels for a predetermined time period such that oxidation of soot accumulated on the filter takes place.
A controlled regeneration can become uncontrolled if the oxidation process drives the temperature of the filter upwards more than is anticipated or desired, sometimes to the point beyond which the filter substrate material can absorb the heat, resulting in melting or other damage to the filter. Less damaging uncontrolled or spontaneous regeneration of the filter can also take place at noxidation temperatures, i.e., when the filter temperature falls between about 250 C and 400 C. Such uncontrolled regeneration generally does not result in runaway temperatures, but can result in only partial regeneration of the soot on the filter. Partial regeneration can also occur when a controlled regeneration cannot continue because of a drop in temperature, exhaust gas flow rate, or the like. Partial regeneration and other factors can result in non-uniformity of soot distribution across the filter, resulting in soot load estimation inaccuracies and other problems.
The temperature of the particulate filter is dependent upon the temperature of the exhaust gas entering the particulate filter. Accordingly, the temperature of the exhaust must be carefully managed to ensure that a desired particulate filter inlet exhaust gas temperature is accurately and efficiently reached and maintained for a desired duration to achieve a controlled regeneration event that produces desired results.
Conventional systems use various strategies for managing the particulate filter inlet exhaust gas temperature. For example, some systems use a combination of air handling strategies, internal fuel dosing strategies, and external fuel dosing strategies. The air handling strategies include managing an air intake throttle to regulate the air-to-fuel ratio. Lower air-to-fuel ratios, e.g., richer air/fuel mixtures, typically produce higher engine outlet exhaust gas temperatures. Internal fuel dosing strategies include injecting additional fuel into the compression cylinders. Such in-cylinder injections include pre-injections or fuel injections occurring before a main fuel injection and post-injections or fuel injection occurring after a main fuel injection. Generally, post-injections include heat post-injections and non-heat post-injections. Heat post-injections are injections that participate along with the main fuel injection in the combustion event within the cylinder and occur relatively soon after the main fuel injection. Non-heat post injections are injections that occur later in the expansion stroke compared to the heat post-injections and do not participate in the combustion event within the cylinder.
In internal combustions engines, unburned fuel can be forced by a combustion event to slip past, e.g., blow-by, the seals between the piston head and the wall of the compression cylinder. The unburned fuel that slips past the seals enters the crankshaft case chamber below the compression cylinders and intermixes with, e.g., dilutes, lubricating oil stored in the chamber. The fuel dilution level of an engine then is a measure of unburned fuel in the lubricating oil in the crankshaft case (often expressed as the percentage of unburned fuel in the fuel/oil mixture). Most engines generate normal amounts of fuel dilution (e.g., less than about 3%-5%), which often evaporates from the heat of the engine without negatively affecting the engine. However, when fuel dilution levels reach above-normal levels, the fuel does not burn off and may excessively thin the oil. Fuel diluted oil having excessively high fuel dilution levels can lower the lubricating properties of the oil, which can cause a drop in oil pressure and an increase in engine wear. Therefore, preventing the fuel dilution level of an engine from reaching above-normal amounts is an important part of proper engine maintenance and performance.
Although conventional regeneration fuel injection strategies may be adequate for controlling the temperature of exhaust generated by the engine, they often fail to maintain acceptable fuel dilution levels. For example, conventional systems with one heat post-injection participating in the combustion of fuel within the cylinder results in excessively high fuel dilution levels. Further, conventional regeneration fuel injection strategies result in more than typical amounts of fuel being injected into the compression cylinder. As discussed above, some of this fuel does not participate in the combustion event, i.e., the fuel is not combusted, and is not vaporized. With more fuel being injected into the compression cylinder than can be combusted and less vaporization of the fuel, the cylinders often contain excessive amounts of unburned and unvaporized fuel, which typically leads to increased fuel dilution levels.
Based on the foregoing, a need exists for a fuel injection strategy that achieves targeted engine outlet exhaust for desired regeneration events while maintaining fuel dilution levels at or below an acceptable level for the engine.